1. Field of the Invention
The present invention relates to a drive power transmission device for use in a motor vehicle, and more particularly to a drive power transmission device for use in a four-wheel drive motor vehicle.
2. Description of the Relevant Art
One known central differential for a four-wheel drive motor vehicle is a control device for an automotive transfer assembly disclosed in Japanese Utility Model Publication No. 56-55055, for example. The control device has a planetary gear mechanism essentially comprising a sun gear, plant gears, and a ring gear.
Another central differential heretofore proposed is a viscous shear coupling disclosed in U.S. Pat. No. 4,022,084. The disclosed viscous shear coupling comprises coaxial input and output shafts, an enclosure disposed between the input and output shafts, and first and second groups of plates housed in the enclosure and coupled respectively to the input and output shafts. The plates of the first and second groups are mutually interleaved or alternately arranged in the enclosure. The enclosure is filled with highly viscous silicone oil. Drive power from the input shaft can be transmitted to the output shaft by shearing forces of the silicone oil present between the first and second plates.
When steering a certain four-wheel drive motor vehicle into a garage or a parking space at low speed, difficulty has heretofore arisen in maneuvering the motor vehicle because of the difference between the angular speeds of front and rear wheels. Where a central differential is employed in a four-wheel drive motor vehicle, it can absorb the difference between the angular speeds of front and rear wheels to allow the motor vehicle to turn smoothly, for thereby eliminating the problem of tight turn braking which would otherwise be caused by direct coupling between the front and rear wheels.
For anti-skid control of the front and rear wheels of such a four-wheel drive motor vehicle when it is braked, it is preferable that the front and rear wheels be controlled independently of each other without limiting the differential movement of the front and rear wheels. When starting the motor vehicle on a slippery road such as a snow-covered road, on the other hand, the differential movement of the central differential should be limited for effective transmission of drive power. This can prevent front or rear wheels from excessively rotating idly, making the motor vehicle highly capable of running over rough terrain or the like. Moreover, while the motor vehicle is running normally, more drive power should be distributed to the front wheels than to the rear wheels for greater stability of the motor vehicle since lateral forces applied to the tires start being reduced earlier for the front wheeels than for the rear wheels, thus allowing the driver to predict a reduction in the lateral forces on the rear wheels.
However, the conventional central differentials have failed to meet the above different requirements or functions.